Homoeosis in Drosophila: anterior and posterior transformations of Polycomb lethal embryos.

Lethal embryos homozygous for Polycomb (Pc) mutations show transformations of segment-specific cuticular features to those of more anterior or posterior segments; the frequency and extent of such changes show differences which depend on the genotype and the region. The mesothorax of Polycomb lethal embryos often shows posterior transformations of the anterior- and posterior-most portions of the segment, and anterior transformations of the medial portion. A comparison of Polycomb embryos also bearing various genetic lesions of the bithorax gene complex (BX-C) shows that the penetrance of anterior transformation and the extent of posterior transformation in the appears independent of posterior transformation, even though cells undergoing each of these changes lie in close proximity in the developing embryo. It has been shown previously that in Polycomb lethal embryos posterior transformations require the normal function of the BX-C. We show here that anterior transformations of the mesothorax and other segments require the normal function of the Sex combs reduced (Scr) locus, also necessary for the normal development of the prothorax and some head segments. Similar observations are also presented for a Polycomblike mutation. We suggest that in Polycomb embryos there are errors in the clonal transmission of determined states resulting in expression of the BX-C and Scr+ loci at abnormal locations, and that such events are probabilistic in nature and show marked regional differences in frequency.